This invention relates to a fuel control system and in particular to a fuel control system for use in controlling the supply of fuel to an aircraft engine.
A typical fuel control system for use in controlling the supply of fuel to an aircraft engine comprises a metering valve operable to control the rate at which fuel passes from a supply line to a delivery line. A pressure drop control arrangement, for example comprising a pressure drop control valve and an associated spill valve, is operable to maintain a substantially constant pressure drop across the metering valve. A pressure raising and shut-off valve (PRSOV) controls the passage of fuel from the delivery line to a burner manifold, the PRSOV serving, in use, to maintain a minimum fuel pressure in a part of the fuel control system upstream thereof, so as to ensure that any fuel pressure operated devices arranged to receive fuel under pressure from the fuel control system can operate correctly.
The fuel control system receives fuel under pressure from a pumping unit that is driven by, and so operates at a speed related to the operating speed of, the associated engine. There is a need to provide, within the fuel control system, a mechanism whereby thrust can be controlled in the event of an overthrust condition arising. A number of thrust control management, commonly known as thrust control malfunction, (TCM) systems to perform this function are known. For example U.S. Pat. No. 6,745,556 describes a TCM system in which a control valve is operable to relieve the fuel pressure applied to one end of a metering valve with the result that the metering valve moves to a low or minimum fuel delivery position to allow fuel supply to the associated engine burner to be reduced in the event that an overspeed condition is sensed. The control valve, in this arrangement, is a torque motor controlled valve.
Rather than control thrust by altering the operation of a metering valve when an overthrust condition is sensed, another approach involves modifying the operation of the spill valve so as to limit the fuel flow delivered to the engine via the metering valve and PRSOV, or to use other control valves to limit this flow. For example, one of the arrangements described in U.S. Pat. No. 7,137,242 uses ports provided on a staging valve to allow the pressure at the inlet of the metering valve to be reduced in the event of an overspeed condition being sensed. U.S. Pat. No. 6,619,027 describes an arrangement in which an electrically controlled servo valve allows the operation of the spill valve to be modified, and in which the operation of a shut-off valve can be controlled.
U.S. Pat. No. 5,927,064 describes a fuel system incorporating an overspeed governor to control the operation of a spool valve in response to engine speed and thereby control or manage overspeed conditions.
As well as permitting thrust control, another requirement of the fuel control system is to ensure that sufficient fuel can be delivered to the engine under windmill relight conditions in which the engine is rotating only very slowly, being driven by the airflow incident thereon.
As mentioned hereinbefore, the pumping unit is driven for rotation at a speed related to the operating speed of the associated engine. During windmill relight conditions the engine speed maybe as low as, for example, 5% of its maximum operating speed. At such low operating speeds, parasitic losses within the fuel control system and in the pumping unit itself may result in the pumping unit being unable to deliver sufficient fuel, in the time available, to permit relighting of the engine. In order to increase the fuel delivery rate, it is known to use a larger displacement pump, but the required increase in size and weight of the pumping unit makes this unattractive.
Rather than increase the pump output to ensure that the required flow to the burners is achieved which increases the parasitic leakage losses in the system, another known technique involves adjusting the system pressure rise during start-up and windmill relight conditions, for example by modifying the pressures applied to the PRSOV so that the PRSOV will open at a lower pressure during start-up and windmill relight conditions and the parasitic leakage losses will be reduced. The PRSOV reverts to its normal mode of operation under other engine operating conditions. For example, U.S. Pat. No. 6,321,527 describes arrangements in which the pressure acting upon a minimum pressurising valve urging it towards a closed position is dependent upon the operation of either a solenoid operated control valve or the operation of a pressure regulating valve. Other arrangements in which the operation of the PRSOV is modified to allow a reduction in system pressure rise at start-up or under windmill relight conditions are described in U.S. Pat. No. 7,234,293 and U.S. Pat. No. 6,176,076.